Can anyone tell me the exact dimensions of a 70w bulb, I need it for a little DIY project of mine.

I only need how long the bulb is and how wide it is.

it's about 4.5inch by 1in. save yourself the trouble and buy the $100 70W MH retrofit from hellolights. your costs will probably come close to that anyway. it will also go well with a 20K ushio.

I'm building a small Mini retro the will supplement 24 LED's

i'm not sure about this but i think the metal halides will overpower the leds.

I wouldn't mount the LEDs anywhere near the MH. You'll severely reduce the life of the LEDs if you overheat them.

i'm no LED guru but from what i've read, i would imagine the LED's you could fit in the space the metal halide would take up with outperform the metal halide and not cost any more money, plus you get plenty of the shimmer effect from LED's so no real advantage to adding the halide.